Embryogenesis is initiated by a cleavage stage characterized by rapid mitotic divisions that are under maternal genetic control. During these divisions, there is little or no zygotic gene expression and the simplified cell cycle is composed of alternating S and M phases. Control of embryogenesis switches to the zygotic genome at the midblastula transition (MBT), when the cell cycle slows, zygotic transcription increases dramatically, and G1 and G2 cell cycle phases are introduced. The timing of the MBT is determined by the ratio of nuclei to cytoplasm, suggesting that this transition is triggered as a maternally deposited factor is titrated by DNA or chromatin. Cell cycle checkpoint pathways delay cell cycle progression in response to DNA damage, incomplete replication, or mitotic defects. A conserved DNA replication checkpoint pathway is required for the switch from maternal to zygotic control of development at the Drosophila MBT. This observation supports a model for the MBT in which titration of a maternally deposited DNA replication function triggers the replication checkpoint, which in turn leads to cell cycle delays and the onset of high level gene expression. The long-term goal of this application is to define the molecular mechanism of the transition from maternal to zygotic control of development at the MBT, with particular emphasis on the role of the replication checkpoint pathway at this transition. To address this broad goal, the following specific aims will be pursued. 1) Identify the maternal factors that trigger the replication checkpoint and thus control the development timing of the MBT. 2) Systematically analyze the replication checkpoint pathway that detects titration of these maternal factors and triggers cell cycle delays and activation of the zygotic transcription machinery. 3) Identify the targets for replication checkpoint regulation that mediate zygotic gene activation at the MBT. The Mei-41/ATM tumor suppressor homologue is a component of the checkpoint pathway that controls the MBT. These studies may therefore provide insight into a critical developmental transition, and help define a clinically significant tumor suppressor pathway.